The present invention relates to an etching material, particularly, an etchant for use in etching selectively a layer of an aluminum oxide formed on aluminum, and to an etching process using the same.
Known processes for fabricating semiconductor circuits (ICs) include a technology which comprises using aluminum for the interconnection. An example of the technology is described below with reference to FIGS. 1(A) to 1(D) which illustrate a diagram showing a process for fabricating an electrode or an interconnection 101 based on aluminum on an appropriate base made of an insulator, an insulating film, a semiconductor, etc. Referring first to FIG. 1(A), an electrode or an interconnection 101 containing aluminum as the principal component is formed on a base 100 by means of evaporation and the like. Then, an oxidized aluminum layer (oxide layer) 102 is formed by effecting anodic oxidation in an electrolytic solution using the electrode or the interconnection 101 containing aluminum as the principal component as an anode. The oxide layer 102 is utilized to improve the withstand voltage or to form an offset region of a thin film transistor.
After forming the oxide layer 102, a silicon oxide layer or a silicon nitride layer is formed as an interlayer dielectric 103. A proper mask (generally, a resist) 107 is used to form a contact interconnection with the electrode or the interconnection 101, and etching is performed to form a perforated hole 104 as shown in FIG. 1(C).
The step above is effected by using a buffered hydrofluoric acid or pure hydrofluoric acid as an etchant. Upon completion of the perforation, a proper metallic interconnection is formed by means of vapor deposition or sputtering to form a contact for the electrode or the interconnection 101.
In a practical fabrication process, however, etching proceeds as such to yield an etched state 106 as illustrated in FIG. 1(D). This leads to a problem of forming a deeply etched portion on a part of the electrode or the interconnection 101.
The problem above is attributed to the fact that, when etching is performed using a hydrofluoric acid-based etchant, the etching rate of the aluminum oxide layer 102 becomes far lower than that of the electrode or the interconnection 101 containing aluminum as the principal component.
The problem above can be overcome by a technique which enables selective removal of the aluminum oxide layer. For example, a technique capable of selectively removing the aluminum oxide layer by using an etchant is required. A mixed solution of phosphoric acid and water, with about several percents of chromic anhydride and caustic soda added therein is known as an etchant for use in the etching of an oxide layer formed by anodic oxide using aluminum as the anode.
However, the solution above contains water at a large quantity. Accordingly, the composition of the etchant tends to fluctuate during the etching step, because water vaporizes due to heating (in general, the solution is heated to about 65.degree. C.). Therefore, because of the problems concerning reproducibility and stability, the technique above is practically unfeasible in a semiconductor fabrication process where a precise control in the level of several hundreds of angstroms (.ANG.) is required.
Furthermore, there are cases in which a porous oxide layer alone is removed after forming a dense oxide layer and a porous oxide layer around the electrode or the interconnection containing aluminum as the principal component. In such cases, an etchant which enables etching of the porous oxide layer at a higher etching rate is necessary.
Concerning the well-known etchant comprising a mixed solution of phosphoric acid and water with chromic anhydride added therein at a quantity of several percents, the etching rate for a porous oxide layer (aluminum oxide layer) does not considerably differ from that for a dense oxide layer. This requires an etching process under severely controlled conditions, and hence, leads to problems considering the stability and the yield of the operation. Conclusively, it was found practically difficult to selectively remove the porous oxide layer alone.